This study examines the psychological and neurophysiological basis of the human's ability to detect small temperature changes superimposed upon noxious levels of thermal stimulation. In the first part of the study we examined the relationship between the reaction time to small temperature changes and the ability of humans to estimate the magnitude of the pain sensation. These data show that there is a close correspondence between reaction time and the magnitude estimate of the pain sensation. In addition, even when the reaction time has plateaued and further increases in stimulus intensity do not produce faster reaction times, there are still increases in the magnitude estimates of thermal pain. The responses of cortical nociceptive neurons to small temperature changes superimposed upon noxious levels of thermal stimulation were examined. Manipulations that produce changes in the intensity of pain sensation in humans also produced concomitant changes in the discharge of cortical nociceptive neurons. Therefore, we conclude that cortical nociceptive neurons may participate in the sensory-discriminative aspects of pain.